Polyphenylene ethers are a class of polymers which are widely used in industry, especially as engineering plastics in applications which require such properties as toughness and heat resistance. For many such applications polyphenylene ether resins are blended with various types of polystyrene resins to improve their processability. Recently it has become necessary to produce such compositions which are both substantially odorless and tasteless for food contact applications. It is therefore necessary that the polyphenylene ether resin blend composition be substantially free from any volatile materials which have undesirable odors or would otherwise harm the food. Materials of this kind which are known to be present in polyphenylene ether resin blends include toluene, dialkylamines such as di-n-butylamine, which are components of the catalyst used in preparing polyphenylene ether resins; and volatiles from second thermoplastic resins such as styrene monomers from degradation of any styrene resin; and other by-products resulting from polyphenylene ether resin synthesis. In the case of poly(2,6-dimethyl-1, 4-phenylene ether); 2,4,6-trimethylanisole, 7-methyldihydrobenzofuran, 2,3-dihydrobenzofuran, 2,6-dimethylcyclohexanone and 2-ethylhex-2-enal are commonly present. Removal of sources of volatile odoriferous amines and oxygenated species is especially important because they are detectable in human organoleptic tests even in very low quantities.
Methods are known, to those skilled in the art, for removing volatile substances from polyphenylene ether or polyphenylene ether/polystyrene resins during vented extrusion, with or without the injection of water into the melt. For example, Kasahara et al., U.S. Pat. No. 4,369,278 extrudes polyphenylene ether and rubber reinforced polystyrene in a single pass, single stage vacuum vented extruder with optional water injection; Newmark, U.S. Pat. No. 3,633,880, extrudes plastic materials, which could include polyphenylene ether, in a single pass through an extruder with multiple vents and employs a specially designed screw to compress and decompress the melt aiding devolatilization without employing water. Commonly assigned pending U.S. patent application, Ser. No. 156,046 filed Feb. 16, 1988, extrudes polyphenylene ether in a single pass single stage system using up to 15 percent water and vacuum venting. Although all three of the above-mentioned methods reduce the amount of volatile substances in the resin, none of them reduce the amount of volatile odoriferous species down to a level where such materials are hardly detectable by analytical methods thus providing that articles made from them are substantially odorless, especially in food packaging applications.
Also to be mentioned is Skidmore, U.S. Pat. No. 3,742,903 which describes multiple stage addition of a liquid under pressure where the liquid is maintained under pressure and taken off as a liquid, to separate a substantially insoluble liquid from a feed mixture containing rubber and a polymer.
Ribbing et al., U.S. Pat. No. 4,746,482, disclose degasifying a polyphenylene ether in a degasification extruder and then adding at least one additional molten polymer and blending the degasified molten polyphenylene ether and the additional molten polymer in a melting extruder.
None of the above-mentioned references teach the unexpected result of the substantial reduction of volatile compounds obtained when a second thermoplastic resin is added at a point on an extruder downstream of the addition of the polyphenylene ether and at least one stage of water injection followed by vacuum venting.
Surprisingly it has now been found that sources of volatile odoriferous amines and oxygenated species and other odoriferous compounds are substantially removed from polyphenylene ether resins blended with other thermoplastic resins by the practice of this invention. These very low odor compositions are useful in food packaging application.
Also of interest are Ives, U.S. Pat. No. 4,504,338 disclosing a vented extruder for removing acetone from polyether sulfone resins; and Brown, U.S. Pat. No. 4,806,298 which discloses inactivating and removing amino nitrogen from polyphenylene ether resins to improve impact strength by extrusion with vacuum venting.